How to make holly more bushy

[Rose1]

I have a few holly bushes in ground but they are not growing fast. They have less leaves...... help me!

[applestar]

I thought you were asking about pruning them when I saw the title....

If they're not growing well you need to review the growing conditions. 1st, what variety are they?
I have 4 species of Ilex and they all need different conditions and care.